S.B.K. is short for Shit brown kryptonite, most commonly associated with ones significant other.
unfortunatly that other is not too considerate of the individuals feelings and often ends up destroying them from the inside.
Also shit brown is the combonation of both the colors red and green, like the two types of kryptonite, meaning that this significant other would empower you beyond your regular human capabilities, almost like an adreneline rush, but he/she also has the ability to control you, and destroy you, and usually does.
You do all that you can to have your S.B.K, but your S.B.K doesnt want you like you want them.
It is your ultimate demise disguise as the one you love.

guy 1: dude i just came from my S.B.K. home.
guy 2: are you alright man
guy 1: *collapses on ground and suffers breakdown after breaking down a concrete wall with his own brute force*
guy 2: dammit thats the ninth time this month, get over her.
guy 1: immposible.